pid

This field contains a unique process identifier referred to as PID. PIDs in Linux are of the type pid_t (integer). Though a PID is an integer, the default maximum number PIDs is 32,768 specified through the /proc/sys/kernel/pid_max interface. The value in this file can be set to any value up to 222 (PID_MAX_LIMIT, approximately 4 million).

To manage PIDs, the kernel uses a bitmap. This bitmap allows the kernel to keep track of PIDs in use and assign a unique PID for new processes. Each PID is identified by a bit in the PID bitmap; the value of a PID is determined from the position of its corresponding bit. Bits with value 1 in the bitmap indicate that the corresponding PIDs are in use, and those with value 0 indicate free PIDs. Whenever the kernel needs to assign a unique PID, it looks for the first unset bit and sets it to 1, and conversely to free a PID, it toggles the corresponding bit from 1 to 0.